1. Field of the Invention
This invention relates generally to fluidic gas metering systems for metering the flowrate of a gas stream being supplied to a user, and in particular to a system of this type which is accurate throughout a broad range, yet exhibits a low pressure drop throughout this range.
2. Background of the Invention
(Status of Prior Art)
The supply of natural gas to a residential, commercial or industrial user must be accurately metered in order to be able to calculate the exact charge to be imposed on the user for the amount of gas he has consumed. The range of the metering system must be sufficient to accommodate the gas requirements of the user, and the system must not produce an excessive pressure drop in the gas line. The reason a low pressure drop is an important desideratum is that the metering system is interposed in a gas line extending from a gas supply source to the site of the user and therefore acts as an impedance in the line that resists gas flow.
A preferred type of gas metering system makes use of a fluidic flowmeter having a broad range such as the xe2x80x9cFluidic Flowmeter with Large Flow Metering Rangexe2x80x9d disclosed in the U.S. patent to Kang et al. U.S. Pat. No. 5,239,695 (1994). The main component of a fluidic flowmeter is a fluidic oscillator which generates periodic fluid pulses at a repetition rate proportional to the flowrate of the fluid being metered.
Also disclosing fluidic gas flowmeters are the following U.S. patents:
A. U.S. Pat. No. 5,309,770 (1994) to Okabyashi
B. U.S. Pat. No. 5,335,553 (1994) to Ueki et al.
C. U.S. Pat. No. 5,353,704 (1994) to Huang
Among the many advantages of a fluidic gas flowmeter are the following:
1) The meter is linear throughout its operating range. Hence the repetition rate or frequency of output pulses of the meter is proportional to the flowrate of the gas being metered.
2) The meter has no moving parts and is therefore unaffected by shock and vibrating forces.
3) The meter has a high degree of rangeability.
4) The meter can be calibrated in terms of volumetric flow unaffected by changes in density.
5) The meter does not require repair or maintenance.
The geometry of a standard fluidic flowmeter is such that when a stream of gas is injected into the space between two inclined side walls to initiate the operation of the meter, then because of a Coanada effect, the stream attaches itself to one of these walls, a portion of the flow being diverted through a feedback passage to a control port. This feedback flow enlarges a separation bubble which peels the stream away from the wall to which it is attached, until it locks onto the opposite wall where a similar feedback action takes place. Hence the stream oscillates between the walls and does so at a frequency proportional to its flowrate.
Associated with this fluidic oscillator is a transducer which converts the fluidic pulses to periodic electrical pulses whose repetition rate is proportional to the instantaneous flowrate of the gas. By totalizing these pulses one obtains an accurate indication of the total flow usually expressed in terms of liters of gas consumed by the user.
The quantity of gas metered by the system, when expressed in liters per hour, is equal to the number of pulses yielded by the fluidic oscillator in the specified period multiplied by the amount of gas contained in a single pulse.
It is not only essential that a gas metering system be linear throughout a broad flowrate range, but it is also vital that the pressure drop produced by the system in the line supplying the gas to a user site be low throughout the entire range. Thus should a system exhibit a low pressure drop except in the upper region of the range, it wold not be acceptable to a gas supply company which must accurately meter the gas it supplies to its customer and cannot tolerate a high pressure drop.
The basic requirement that a metering system be accurate throughout a broad flowrate range is not difficult to satisfy with meters using fluidic oscillators. But the requirement that the same system exhibit a low pressure drop throughout its entire range is troublesome. Existing fluidic meters have a more or less low pressure drop when operating in a broad flowrate range except in the upper region of the range where the pressure drop exceeds an acceptable level.
Inasmuch as a fluidic metering system in accordance with the invention includes a bypass passage, of prior art interest in U.S. Pat. No. 5,610,162 (1986) to Okabayashi et al. The fluidic metering system disclosed in this patent includes upstream and downstream fluidic elements, one having a jet nozzle whose opening area is smaller than that of the jet nozzle in the other element. A bypass passage having a valve therein is disposed in parallel with the element which has a small area nozzle. This patent points out that the smaller the area of the jet nozzle opening, the greater is the sensitivity of the meter but the larger is its pressure drop.
In view of the foregoing, the main object of this invention is to provide a fluidic gas metering system which accurately measures the flowrate of a gas stream being supplied to the site of a user throughout a broad range, the system producing a pressure drop which is low throughout its entire range.
More particularly, an object of the invention is to provide a system of the above type in which the flowrate range is divided into low, medium and high flowrate bands, each band being handled by a separate channel, with only two of these channels having a fluidic generator therein. The channel arrangement such that one of the two generators measures flowrate in both the medium and flowrate bands.
Also an object of this invention is to provide a metering system of the above type, which can be manufactured at relatively low cost to produce a compact, self-sufficient unit that is efficient and reliable in operation.
Yet another object of this invention is to provide a gas metering system which is compensated for variations in ambient temperature, barometric pressure or other variables which in the absence of compensation render the meter readings somewhat inaccurate.
Still another object of this invention is to provide a fluidic metering system whose operation is controlled by a microprocessor which acts to process the pulse data supplied thereto by two fluidic generators to provide a reading of the gas consumed by the user that is accurate throughout a broad range.
A significant feature of a system in accordance with the invention which includes a microprocessor is that the digital data acquired by the microprocessor can be microwave transmitted to a customer billing station, to an Internet web site or to any external station in need of this data.
Briefly stated these objects are attainable in a fluidic gas metering system interposed in a line running from a gas source to a user site to meter the gas stream being supplied to the user in a broad flowrate range defined by a low flowrate band at the lower end of the range, a medium band at the middle of the range and a high band at the upper end thereof. The system includes an input chamber which receives a stream of pressurized gas from the source and an output chamber from which the gas is supplied to the user.
Intercoupling the chambers are three gas flow channels. The first channel is defined by a fluidic generator operative only when the flowrate of the gas lies in the low band to yield periodic pulses whose frequency is proportional thereto. The second channel is defined by a fluidic generator operative only when the flowrate of the gas lies in the medium band to yield periodic pulses whose frequency is proportional thereto. The third channel is defined by a bypass passage operative only when the stream lies in the high band, the bypass acting to divide the gas stream between the second and third channel whereby the second generator then meters flowrate in the medium band. The periodic pulses yielded by the first and second generators are processed to provide accurate readings, of the gas consumed by the user throughout the full range.